Hello!
To simplify this expression, you must distribute the term 3x^2 to the expression:
7x^2 +2x -4
To do this, multiply 3x^2 to each term:
3x^2*7x^2=21x^4
3x^2*2x=6x^3
3x^2*-4=-12x^2
Then, add the sums together to get your final answer:
21x^4+6x^3-12x^2
Hope this helps, and have a great day!